High electron mobility transistors (HEMTs) based on gallium nitride (GaN) are used for power and radio-frequency applications. An HEMT has a two-dimensional electron gas (2DEG) which forms in the GaN layer near the interface with an AlGaN barrier where electrons have a very high mobility. The 2DEG forms because of two basic principles: (1) charge due to spontaneous polarization of the GaN layer and (2) piezoelectric polarization due to the lattice mismatch between the GaN and AlGaN layers. The Al content and thickness of the AlGaN layer determine the extent of piezoelectric polarization.
HEMTs are typically normally-on devices because of the automatic formation of the 2DEG. However, power devices are usually implemented as normally-off devices. To be normally-off, the 2DEG of an HEMT must be interrupted between the source and drain of the device. One way to realize a normally-off HEMT is to recess the gate into the AlGaN barrier layer, extinguishing the 2DEG under the recessed region. While such a structure yields a normally-off structure, the recess process must be precisely controlled e.g. in the order of only 1 nm. Otherwise a large spread in the threshold voltage results due to a variable thickness of the AlGaN layer under the gate. In addition, the gate must be isolated from the AlGaN material to avoid large gate leakage which can result from the lowered gate Schottky barrier. The recessed gate structure also yields a relatively low threshold voltage which is undesirable for power applications.
Another way to realize a normally-off HEMT is to form the gate from p-doped GaN material. A normally-off HEMT with a p-doped GaN gate typically has a threshold voltage of about 1.5V, but the AlGaN barrier must be thin and have low Al content which negatively affects the on-resistance due to the reduced carrier density in the 2DEG. These limitations arise because of difficulties associated with realizing high doping densities in p-doped GaN and limitations associated with efficiently depleting a 2DEG channel via p-type semiconductor material (even assuming an ideally highly doped p-type layer). Other disadvantages of a normally-off HEMT with a p-doped GaN gate include lowered transconductance and limited gate voltage since the p-doped gate forms a pn-junction which begins to conduct at a relatively low positive gate voltage of about 5 to 6V.
A normally-off HEMT can also be realized by implanting fluorine into the 2DEG channel region. Such a structure has a threshold voltage of about 1V, but there are unsolved technological issues with this approach such as the stability of the implanted species, temperature dependency and aging. Other approaches may be used to manufacture a normally-off HEMT. In each case, an electric field is used to control the 2DEG channel.